aboveH20 --
If I were able to go solo to WDW, I would have a fantastic time!
I almost feel guilty admitting how much I enjoy solo trips!

While I can't deny the fun of being with people whose company I enjoy and just acting silly, laughing all day, as well has having friends to enjoy rides and meals with, there is something so liberating and invigorating about being solo in the parks, isn't there? I absolutely love it. I never, in a million years, would have thought that I'd be doing solo DLR trips if someone had mentioned the concept to me, say, 10 years ago or 30 years ago or whatever. But now... I cannot imagine
not doing them. I like to have a nice balance of spending a bit of "friend time," and then the rest is all solo time. Solo trips are so amazing for those who either love to take a lot of photos, or soak in all of those great details -- or both.
I think that Storytellers sounds like a great way to end your DLR visit!
Let's see -- I've never dealt with the train station near DLR, though I have read other people's reports that involved going there and it seemed like it was not too far away. If it really is, indeed, only 4 miles away from DLR, it shouldn't take more than 15 minutes or so to get there, give or take, depending on traffic. Probably much less than that. I don't know what traffic is like in that specific area though, so I wouldn't want to cut it too close.
If I were in your shoes, I would probably feel safe leaving the GCH at least 30 minutes ahead of the train departure time (so 10:50-ish), just to allow for traffic and any other issues I might need to deal with at the station.
I have had experience with adding people to my reservations, at the GCH and at the PPH. It should not be a problem at all for them to add your son to the reservation. I recommend doing it at check-in, if not before you arrive. I usually call at some point before my arrival date and add any potential guests to the reservation. This way they will have the guests' names already in the file and give me all the keys when I check in.
There is a charge for parking if you need it (no charge if you don't need it), so the GCH will charge you for that if you tell them that a member of your party will have a car. Make sure you find out from them if they will charge you parking for every night you're at the GCH (even on the days when your son is not there), or only the nights when your son joins you. I remember thinking that I would only be charged for the specific day the parking was needed, but I was told that I would be charged for every night of the trip -- even if the guest only needed to park one day.
The extra key for your son won't be a problem. Chances are you will be given a room that can accommodate "up to 5 people." So as long as you don't exceed a total of 5 people in your room, you can add guests to the reservation for no extra charge other than the parking.
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